Gas-dynamics of laser ablation: Two-dimensional expansion of the vapor in an ambient atmosphere

A two-dimensional (2D) gas-dynamic model of laser ablation in an ambient gas atmosphere is proposed. To obtain the boundary conditions at the evaporated target surface, a nonlinear heat transfer problem in the target including the dynamics of the melt and evaporation fronts is considered. Back condensation of the vapour at the target is taken into account. At later stages, complete absorption of the vapour and back condensation thereof with a local sound speed are assumed. The gasdynamic problem is divided into the initial one-dimensional (1D) and final 2D stages. The 1D stage describes the ablation plume formation under the action of laser pulse (duration of the order of 10 ns; fluence about several J/cm(2); laser spot diameter about 1 mm). The 2D stage is responsible for the formation of the energy and angular distributions of the ablated material. A considerable compression of the ambient gas around the expanding plume of the laser-evaporated material and a shock front propagating through the undisturbed ambient gas are found. The pressure of the compressed ambient gas behind the shock may be much higher than the ambient one. Once the laser pulse is over, the vapour pressure eventually drops down to the value comparable to the compressed ambient gas pressure. From this time on, the gas considerably suppresses the vapour expansion. There is a noticeable difference between the vapour distribution in vacuum and the one in the ambient atmosphere: the vapour fills the entire plume volume in vacuum while in the presence of ambient atmosphere it is accumulated near the plume boundary and tends to form a thin shell. The angular and energy distributions of the ablated material are especially sensitive to the nature and pressure of the ambient gas. Both the kinetic energy of the ablated atoms and the width of their angular distribution decrease with the ambient pressure.